Duck introductions cover the range from instant happiness to instant conflict.
What worked for us (two adult Buffs added to a flock of nine Runners) was to bring the Buffs home in the afternoon just before night-night. We had already set up a "room" next to the night pen in the walkout basement with bedding, water, food and oyster shell. We brought the Buffs into their pen, then brought the Runners in. They could see each other with a fence between.
A little posturing, some chatting, everybody settled down soon.
Next morning, Runners went out to the fenced garden as usual. I set up a smaller area next to it, and brought the Buffs out. Things looked good, so after a while I opened the fence up. Some pushing, shoving, little bit of half-hearted chasing.
Things progressed just fine and within a day or two the Runners would not go anywhere without the Buffs.
Ours are all female, which, I believe, made it much easier than a mixed flock.
But that's the general idea - let them see each other across a fence, see how it goes, follow your instincts about how quickly to go to the next step, always standing by to intervene.
I made sure to give everybody treats frequently during the process.